From af845ec01b33054b3771b700aebe4c5bc870dd9e Mon Sep 17 00:00:00 2001
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Fri, 13 Jul 2018 16:58:52 -0700
Subject: [PATCH] mm: do not drop unused pages when userfaultd is running
References: git fixes (mm/userfaultfd)
Patch-mainline: v4.18
Git-commit: bce73e4842390f7b7309c8e253e139db71288ac3
KVM guests on s390 can notify the host of unused pages. This can result
in pte_unused callbacks to be true for KVM guest memory.
If a page is unused (checked with pte_unused) we might drop this page
instead of paging it. This can have side-effects on userfaultd, when
the page in question was already migrated:
The next access of that page will trigger a fault and a user fault
instead of faulting in a new and empty zero page. As QEMU does not
expect a userfault on an already migrated page this migration will fail.
The most straightforward solution is to ignore the pte_unused hint if a
userfault context is active for this VMA.
Link: http://lkml.kernel.org/r/20180703171854.63981-1-borntraeger@de.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/rmap.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 604fefd499f0..91f9df2567f1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -64,6 +64,7 @@
#include <linux/backing-dev.h>
#include <linux/page_idle.h>
#include <linux/memremap.h>
+#include <linux/userfaultfd_k.h>
#include <asm/tlbflush.h>
@@ -1492,11 +1493,16 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
set_pte_at(mm, address, pvmw.pte, pteval);
- } else if (pte_unused(pteval)) {
+ } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) {
/*
* The guest indicated that the page content is of no
* interest anymore. Simply discard the pte, vmscan
* will take care of the rest.
+ * A future reference will then fault in a new zero
+ * page. When userfaultfd is active, we must not drop
+ * this page though, as its main user (postcopy
+ * migration) will not expect userfaults on already
+ * copied pages.
*/
dec_mm_counter(mm, mm_counter(page));
} else if (IS_ENABLED(CONFIG_MIGRATION) &&